Method of manufacturing an active matrix substrate and an image display device using the same

ABSTRACT

The present invention provides a manufacturing method of a high performance active matrix substrate at a high throughput with a less expensive apparatus, and an image display device using the active matrix substrate. On a stage moving in the short axis direction X and long axis direction Y on a rail, a glass substrate is carried, which has an amorphous silicon semiconductor film formed. Polycrystallized and large grain silicon film may be obtained by intensity modulating the pulsed laser beam in a line beam shape by means of a phase shift mask with a periodicity in the long axis direction Y of the laser beam, moving the laser beam randomly in the modulation direction of the amorphous silicon semiconductor film formed on the glass substrate to expose to crystallize the film. The image display device may incorporate an active matrix substrate having active elements such as thin film transistors formed by this silicon film.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional of U.S. application Ser. No. 10/712,712filed Nov. 14, 2003 now U.S. Pat. No. 7,022,558. This application claimspriority to U.S. application Ser. No. 10/712,712 filed Nov. 14, 2003,which claims priority to Japanese Patent Application No. 2003-143803filed May 21, 2003, the contents of which are hereby incorporated byreference into this application.

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing an activematrix substrate having a driver circuit using a low temperaturepolysilicon semiconductor layer and to an image display device using thesame.

BACKGROUND OF THE INVENTION

LCD (liquid crystal display) and EL (electroluminescence display) are intheir development or commercial stage, as image display devices ofso-called flat panel type. Since the liquid crystal display is thin andlower in consumption power, it is used for monitors for personalcomputers and various information processing units, or televisionreceiver sets. The electroluminescence display on the other hand islight emitting type, which does not require any external light sourceused for example in the liquid crystal display devices, allowing makingthinner and light weight image display devices. The active matrix imagedisplay of such devices uses an active matrix substrate having a numberof pixel circuits of matrix arrangement, active elements that drives thematrix circuits, pixel driver circuit that drives pixel circuits, andany other auxiliary circuit necessary for image display formed on a samedielectric substrate. The pixel circuits and pixel driver circuits usesas circuitry elements active elements formed on a silicon semiconductorlayer formed on the dielectric substrate constituting the active matrixsubstrate. The exemplary active element is thin film transistors.

Recently, adopting thin film transistors using low temperaturepolysilicon semiconductor film for those active elements to form thepixel driver and auxiliary circuits by the thin film transistor on adielectric substrate to form an active matrix substrate is realized soas to obtain an image display device with high definition image and lowmanufacturing cost. Herein below low temperature polysiliconsemiconductor film will be referred to as polysilicon film, or sometimessemiconductor film.

A widely used manufacturing method of semiconductor thin film of thetype represented by the polysilicon semiconductor film may be lasercrystallization method, conventionally consisted of forming an amorphoussilicon semiconductor film on a dielectric substrate, exposing theamorphous silicon semiconductor film with laser beam and annealing andcrystallizing. In particular in order to obtain a polysiliconsemiconductor film on a large dielectric substrate such as the activematrix substrate used in the image display device for televisionreceiver set, crystallization of multi-shot method or multi-shot lasercrystallization method is used, in which a plurality of pulse excimerlaser for yielding a higher output power is exposed for crystallization.

Multi shot laser crystallization method may yield a relatively largegrain silicon crystal of the grain size of 0.5 μm or more. This may beuseful for forming a semiconductor film having a high electron mobilitysuitable for the driver circuit contained in the active matrix substrateof an image display device. To obtain a uniform film of large size, itis common that the exposure pattern of the laser beam on thesemiconductor film should be shaped to a rectangular, more specificallyto a line beam in which the short axis width is extremely short withrespect to the long axis width, and the dielectric substrate havingsemiconductor films formed thereon will be moved in relation to theshort axis direction of the shaped beam during the exposure.

In the crystallization method of silicon thin film by the exposure oflaser beam, narrowing the transfer distance of the laser beam during theexposure intervals between two laser pulses much smaller than the beamwidth (that is, the length of short axis) may obtain a good result ofannealing effect. By uniformly forming a laser beam of line beam shapethat has a long width in the long axis direction with respect to thewidth in the short axis direction, crystals may be grown with no gapsfor a large area. Also, as an alternative method of obtaining asemiconductor film of high electron mobility with a larger grain size,crystallization using lateral growth is now being considered.

In the non-patent document 1, Japanese Journal of Applied Physics Vol.31, (1992) pp. 4545-4549, it is disclosed that a larger grain crystalmay be formed by exposing laser beams on an amorphous siliconsemiconductor film while having the thermal capacity of the dielectricsubstrate differed to form a thermal gradient therein to cause growth ofsilicon crystals from the area of lower temperature to the area ofhigher temperature.

In the patent document 1, JP-A No. 140323/1994, it is disclosed a methodthat enlarges the grain size by modulating the excimer laser beam with agrating to expose an amorphous silicon semiconductor film with such amodulated beam to cause a temperature gradient and growth of crystalsfrom the area of lower temperature to the area of higher temperature.

In the patent document 2, JP-A No. 274088/2001, it is disclosed a methodthat a sequential exposure of laser beam with translation of substrateof slight overlap to the previous melt zone to induce sequential growthof crystal in the lateral direction to form a large grain crystal, or itis referred to as SLS method. Another example of sequential exposure ofpulse laser beam with a slight overlap to the previous melt zone isdisclosed in the non-patent document 2, in which the pulsed laser beamis scanned while exposure on the amorphous silicon film. In this methodit is described that the scan speed of the laser beam is 99 centimetersper second, and the frequency of laser beam is 2 kHz. The interval of49.5 μm between two laser beam exposures may be calculated. In FIG. 9 ofthis reference it is described an example in which the melt width byeach pulse of laser beam is more than 50 μm and a melting zone isoverlapped to its previous melted zone.

In the patent document 3, JP-A No. 280302/2002 it is disclosed that alateral growth method for forming a large grain crystal by exposing alaser beam of intensity modulated by the interference of laser beamswhile translating by the growth distance in the lateral direction.Another example of lateral grain growth method using laser modulation isdisclosed in the non-patent document 3. In this paper, a phase shiftmask, which is placed on the amorphous Si film substrate, modulate laserbeam intensity periodically to induce lateral grain growth.

[Non-patent Document 1]

Japanese Journal of Applied Physics Vol. 31, (1992) pp. 4545-4549

[Non-patent Document 2]

Japanese Journal of Applied Physics Vol. 21, (1982) pp. 879-884

[Non-patent Document 3]

Japanese Journal of Applied Physics Vol. 37, (1998) pp. 5474-5479

[Patent Document 1]

JP-A No. 140323/1994

[Patent Document 2]

JP-A No. 274088/2001

[Patent Document 3]

JP-A No. 280302/2002

In order to improve the throughput when forming a large grain crystal bythe above mentioned multi-shot laser crystallization for a semiconductorfilm formed on a dielectric substrate for use in an active matrixsubstrate of an image display device, it is imperative to reduce thenumber of shots of the laser beam. However, if the number of shots oflaser beam is reduced when using a laser beam of uniform intensitywithout any modulation according to the prior art, the grain size ofyielded crystal will be shrunk, resulting in a decrease of electronmobility, so that the improvement of throughput has been difficult.

On the other hand, in the semiconductor film crystallization methodusing the lateral growth, it is possible to decrease the number of shotsof laser beam. However, in any conventional methods the relativeposition of laser beam with respect to the dielectric substrate shouldbe controlled, by the length similar to the growth distance in thelateral direction. The lateral growth distance of a crystal may dependon the silicon film thickness, substrate temperature at thecrystallization, and the pulse duration time of irradiated laser beam.For example when melting and crystallizing a silicon semiconductorsubstrate of film thickness of 50 nm with pulsed excimer laser beam of25 nsec at the room temperature, lateral growth distance of crystalswill be 1 μm or less. Therefore, the exposure position is required to becontrolled at the precision of 1 μm or more. This requires a highlyaccurate translating mechanism, resulting in a higher installation cost.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstancesand has an object to overcome the above problems and to provide a methodof manufacturing an active matrix substrate, allowing a higherthroughput by the promotion of lateral growth of silicon crystal bymeans of a modulated laser beam, and allowing using a less expensivedevice, which incorporates a relative displacement mechanism of thelaser beam to a dielectric substrate of an error larger than the lateralgrowth distance of crystal.

In order to achieve the above object, first feature of the manufacturingmethod of an active matrix substrate in accordance with the presentinvention includes forming a semiconductor film on a dielectricsubstrate, and when crystallizing the semiconductor film by exposing alaser beam modulated periodically on the semiconductor film, movingrandomly the exposure position of the laser beam in the periodicdirection. This allows lateral growth by using an apparatus having anerror of the relative displacement mechanism of the laser beam to thedielectric substrate larger than the lateral growth distance, thusallowing a higher throughput when using a less expensive apparatus.

A second feature of the manufacturing method of an active matrixsubstrate in accordance with the present invention includes dividingzones smaller than the crystal grain of semiconductor film formed on thedielectric substrate, promoting lateral growth using thus dividedcrystal grains as the nuclei to reform a unique crystal grain to allowcrystallization of semiconductor film. This method does not create a newgrain boundary regardless of the relative exposure position of laserbeam with respect to the dielectric substrate, allowing forming a highquality crystal of uniform grain size in combination with theabove-mentioned first feature of random displacement.

A third feature of the manufacturing method of an active matrixsubstrate in accordance with the present invention includes exposing apulsed laser beam modulated periodically so as to have a firstperiodicity, and exposing another laser beam modulated so as to have asecond periodicity smaller than the first periodicity to crystallize thesemiconductor film. This second pass with the second cycle smooths thesurface of semiconductor film while the grain size stays the same. Inorder words, the peaks over hillocks are smoothed out to form a smoothersurface by the second pass. By using this in combination with theabove-mentioned features, a semiconductor film having an isotropiccrystallinity may be obtained with a less expensive apparatus. Also aflat film may be yielded which is appropriate for forming thin filmtransistors.

An feature of image display device in accordance with the presentinvention includes arranging a plurality of pixel circuits on the pixelarea on a dielectric substrate of an active matrix substrate, arranginga pixel driver circuit and auxiliary circuit outside the display area,and being formed by thin film transistors using as channelssemiconductor thin film having these pixel circuit and pixel drivercircuit or other circuits manufactured according to any one of abovemanufacturing methods.

Another feature of image display device in accordance with the presentinvention includes a plurality of wirings intersecting each with otheron the display area of the dielectric substrate forming the activematrix substrate, pixel circuit arranged in the vicinity of theintersection of wirings for varying the transmittance or reflectance oramount of light emission, a thin film transistor including asemiconductor thin film in the pixel circuit used for a channel, thefilm having a switching element manufactured in any one of the abovementioned manufacturing methods for selectively driving the pixel.

Another feature of the image display device in accordance with thepresent invention includes a plurality of wirings intersecting each withother on the display area of the dielectric substrate forming the activematrix substrate, a pixel circuit made of light emitting element s inthe vicinity of the intersection of wirings, a thin film transistorincluding a semiconductor thin film in the pixel circuit used for achannel, the film having a switching element manufactured in any one ofthe above mentioned manufacturing methods for selectively driving thepixel, using a semiconductor film manufactured in any one of the abovementioned manufacturing methods for the channel of the thin filmtransistor selectively driving the light emitting element, so as for thechannel length to be a multiple of natural number of the periodicity ofhillocks of the semiconductor film, with the channel direction inparallel to the direction of periodicity of hillocks of thesemiconductor film.

Additional objects and advantages of the invention will be set forth inpart in the description which follows and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and attained bymeans of the instrumentalities and combinations particularly pointed outin the appended claims.

It is to be understood that the present invention is not to be limitedto the details herein given but may be modified within the scope of theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification illustrate an embodiment of the inventionand, together with the description, serve to explain the objects,advantages and principles of the invention. In the drawings,

FIG. 1 is a schematic diagram of an embodiment of manufacturing methodof an active matrix substrate in accordance with the present invention;

FIG. 2 is a schematic diagram of laser beam intensity distribution usedin the present invention;

FIG. 3 is a schematic diagram of an exemplary scan of the laser beam inthe manufacturing method of an active matrix substrate in accordancewith the present invention;

FIG. 4 is a schematic diagram of an exemplary laser beam intensitydistribution and relationship relative to a substrate at the time oflaser beam scan in the manufacturing method of an active matrixsubstrate in accordance with the present invention;

FIG. 5 is a schematic diagram illustrating the melting of semiconductorfilm formed on a dielectric substrate constituting an active matrixsubstrate at the time of exposure of pulsed laser beam;

FIG. 6 is a cross sectional view and plan view of a siliconsemiconductor film crystallized by exposing laser beam modulated by aphase shift mask to be again exposed by the laser beam modulated by thephase shift mask;

FIG. 7 is a cross sectional view and plan view of a crystal afterexposing laser beam in the arrangement illustrated in FIG. 6;

FIG. 8 is a schematic diagram of an exemplary crystal prior to exposinglaser beam over the melting threshold, to a polysilicon film having agrain size similar to the periodicity in long axis direction, so as todivide a single crystal grain not including grain boundary;

FIG. 9 is a schematic diagram of an exemplary crystal after exposinglaser beam over the melting threshold, to a polysilicon film having agrain size similar to the periodicity in long axis direction, so as todivide a single crystal grain not including grain boundary;

FIG. 10 is a schematic diagram illustrating the arrangement of grainboundary and hillocks of polycrystalline semiconductor film formed inaccordance with the preferred embodiment of the present invention;

FIG. 11 is a microscopic photograph illustrating an exemplary siliconsemiconductor film obtained in accordance with a manufacturing method ofthe present invention;

FIG. 12 is a schematic sketch of boundaries and hillocks of thephotograph shown in FIG. 11;

FIG. 13 is a schematic diagram illustrating an example of process stepsof manufacturing a semiconductor film by exposure of laser beams eachhaving different periodicity;

FIG. 14 is a schematic diagram illustrating another example of processsteps of manufacturing a semiconductor film by exposure of laser beamseach having different periodicity;

FIG. 15 is a microscopic photograph illustrating an exemplary siliconsemiconductor film formed by sequential exposure of laser beams eachhaving a periodicity in perpendicular directions, in accordance withanother preferred embodiment of manufacturing method of the presentinvention;

FIG. 16 is a schematic sketch of boundaries and hillocks of thephotograph shown in FIG. 15;

FIG. 17 is a plan view illustrating another embodiment of circuit designon an active matrix substrate for a liquid crystal display device havingthin film transistors formed on a polycrystallized semiconductor film inaccordance with the manufacturing method of the present invention;

FIG. 18 is a plan view illustrating an arrangement of one pixel part ofa semiconductor device to be used in a liquid crystal display deviceformed by using the semiconductor thin film in accordance with themanufacturing method of the present invention;

FIG. 19 is a cross sectional view taken along the dashed lines A-B ofFIG. 18;

FIG. 20 is a schematic diagram depicting process steps of manufacturingan active matrix substrate shown in FIG. 19;

FIG. 21 is a schematic diagram depicting process steps of manufacturingan active matrix substrate shown in FIG. 19, next to FIG. 20;

FIG. 22 is a schematic diagram depicting process steps of manufacturingan active matrix substrate shown in FIG. 19 next to FIG. 21;

FIG. 23 is a schematic diagram depicting process steps of manufacturingan active matrix substrate shown in FIG. 19, next to FIG. 22;

FIG. 24 is a schematic diagram depicting process steps of manufacturingan active matrix substrate shown in FIG. 19, next to FIG. 23;

FIG. 25 is a schematic diagram depicting process steps of manufacturingan active matrix substrate shown in FIG. 19 next to FIG. 24;

FIG. 26 is a schematic diagram of an exemplary image display deviceincorporating organic electroluminescence elements by means of an activematrix substrate made by the manufacturing method of the presentinvention;

FIG. 27 is a schematic diagram of an equivalent circuitry of the organicelectroluminescence element shown in FIG. 26; and

FIG. 28 is a cross sectional view taken along the line C-D of FIG. 26.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A detailed description of preferred embodiments of the manufacturingmethod of an active matrix substrate and image display device using theactive matrix substrate, embodying the present invention will now begiven referring to the accompanying drawings. Referring to FIG. 1, thereis shown a schematic diagram illustrating a preferred embodiment ofmanufacturing method of an active matrix substrate in accordance withthe present invention. The reference numeral 1 in FIG. 1 designates apulsed laser beam, 2 an amorphous silicon semiconductor film, 3 apolysilicon semiconductor film (also referred to as polysilicon film,polysilicon Si film herein below), 4 a phase shift mask, 5 a stage, 6 adielectric substrate (usually made of glass, and will be referred to asa glass substrate or simply substrate herein below), 7 a mirror, and 8 acylindrical lens.

On a stage 5 translating on a rail in the short axis direction X andlong axis direction Y, a glass substrate 6 is carried on thereto havingan amorphous silicon semiconductor film 2 (also referred to as anamorphous Si film herein below) formed thereon. A pulsed laser beam 1 ina shape of line beam is intensity modified or modulated by means of aphase shift mask 4 periodic in the long axis Y direction of the laserbeam to expose to the amorphous silicon semiconductor film 2 formed onthe glass substrate 6. The phase shift mask 4 has a number of very fineslits formed its mask surface, disposed as a periodic array, which arrayhas its functionality of modulating the phase of transmitted laser beam,and of periodically modulating the intensity due to the interference.The laser beam 1 is adjusted so as to form a beam shape of approximatelyflat fluence in the short axis X direction on the glass substrate 6,i.e., a top flat shape, by the optics including the cylindrical lens 8that serves as light condensing of laser beam in the short axis Xdirection if there is not a phase shift mask 4.

The temperature at laser beam exposure may be at room temperature, N2gas not shown in the figure is supplied to flow between the glasssubstrate 6 and the phase shift mask 4 during the exposure of laser beam1. Also, the mask surface of the phase shift mask 4 and the glasssubstrate 6 are maintained to be leveled in order to maintain theconstant modulation of laser beam on the glass substrate 6. A pulsedexcimer laser beam of wavelength of 308 nm is used for the laser beam 1.

The periodicity of slits of the phase shift mask 4 may be set to be inthe range of 2 μm to 10 μm, for shaping a laser beam having a modulationperiodicity in the range of 0.5 μm to 10 μm. In this preferredembodiment the periodicity of the phase shift mask 4 is set to 3 μm. Thephase shift mask 4 is made of quartz, having striped steps formedperiodically so as to create a phase difference of 180 degrees for thelaser beam of wavelength 308 nm, between two adjacent steps. When theheight of the phase shift mask 4 from the glass substrate 6 is set to be0.9 millimeter, an intensity modulation of the 1.5-μm period, that is, ahalf of the period of a phase shift mask has been obtained. The laserbeam modulation by the phase shift mask 4 may also include othercomponents of period of fractions of the slit period, other than thehalf period above.

The optics for shaping the laser beam 1 to a line beam splits theincoming laser beam into a plurality of beams each having slightlydifferent incident angle and then irradiate the glass substrate 6 on theprocess of magnifying beam length. When emitting such laser beam intothe phase shift mask 4, every modulated components are averageddepending on the height from the glass substrate 6 to the phase shiftmask 4, resulting in an emphasis of a specific periodic component.Therefore when using the arrangement in accordance with the preferredembodiment, modulation of any other period other than a half of slitperiod of the phase shift mask 4 such as ⅔ period may be obtained.

When forming a semiconductor film, the laser beam 1 will be oscillatedat a repeated constant frequency such as at 300 Hz, scans the stage 5 inthe short axis X direction of the laser beam at a constant speed whilemoving randomly in the long axis Y direction to expose the laser beam 1to the glass substrate 6 to crystallize the amorphous siliconsemiconductor film 2 on the glass substrate 6 to form sequentially apolysilicon film 3.

As a method of moving the exposure position in the modulation period,i.e., in the long axis Y direction of the laser beam 1, a translationmechanism of stage 5 in the long axis Y direction may be used toreciprocally move in the admissible tolerance of irradiation error forexample on the order of 0.1 millimeter. The translation of the stage 5usually is accompanied by a back rush of the order of micrometers,causing an error of the level comparable to the modulation period of thelaser beam, during a reciprocation, resulting in a randomized amount oftravel distance. For moving randomly, a swell of the amplitude of theorder of few microns is formed on the rail to travel the stage 5 on theswell. Alternatively instead of moving the stage 5, a mechanism ofvibrating the phase shift mask 4 may be provided. In addition, byoscillating the angle of the mirror 7 that reflects the laser beam 1incident to the phase shift mask 4, the incident angle of laser beam maybe oscillated in relation to the modulation direction of the phase shiftmask 4 so as to displace the intensity distribution of the laser beam 1.

Referring to FIG. 2, there is shown a schematic diagram of intensitydistribution of laser beam used in accordance with the presentinvention. The beam shape of the laser beam 1 may be a very elongatedrectangular, i.e., substantially a line form, for example, of 370millimeters of width 12 in the long axis Y direction, 360 μm of width 11in the short axis X direction. In the long axis direction of line beamof the phase shift mask 4 a periodic pattern is formed. The laser beam 1having passed through the phase shift mask 4 may have a periodicintensity modulation in the Y direction formed. On the other hand, a topflat form is formed in the short axis X direction.

As in the present embodiment, by placing the periodic direction of thephase shift mask 4 in parallel to the long axis of the laser beam, thediffracted light beam by the phase shift mask will be diffracted intothe long axis direction, and will remain within the line beam, resultingin that it has an advantage of avoiding the loss of laser beam 1. Also,since there may be no diffraction of the laser beam 1 in the short axisX direction, and the beam will remain in the form of top flat prior totransmission of the phase shift mask 4 in the short axis X direction, sothat the mean fluence at the time of exposure of each shot can beapproximately constant, resulting in that it has an advantage ofsuperior process stability. In addition, as the laser beam 1 may arrivefrom the glass substrate 6 at the incident range of approximately 180degrees, so that the optics can be considered to have a numericalaperture of 1 in the long axis Y direction, yielding a resolution ashigh as the wavelength, and very precipitous modulation.

Referring to FIG. 3, there is shown a schematic diagram illustrating anexample of laser beam scanning method in a manufacturing method of anactive matrix substrate in accordance with the present invention. InFIG. 3, there is shown an example of crystallization of entire surfaceof the glass substrate 6 that has approximately two times of the lengthof long axis of the laser beam 13. A laser beam of a line beam shapemodulated in the long axis direction is moved at a constant velocity inthe short axis direction of the laser beam 13 on the glass substrate 6while displacing randomly in the long axis direction of the beam duringexposure. By setting a smaller displacement distance between two laserpulses in the short axis direction than the laser beam width, an overlapbetween two exposure zones may be provided so as to irradiate the laserbeam a plurality of times on the same area.

The number of exposure of the laser beam may be controlled by thedisplacement velocity toward the short axis direction. In FIG. 3, afterhaving exposed the top half of the glass substrate 6, the bottom halfthereof will be exposed in the same manner to obtain a substrate havingentire surface crystallized. By arranging the irradiation area of thetop half to be overlapped to the bottom half, a seamless crystallizationcan be obtained on the entire substrate. In this preferred embodiment,even when the position of exposure for the top half is moved withrespect to the bottom half, the random displacement in the long axis Ydirection will cancel the nonuniformity of crystallization, resulting inthat there will be an advantage that no degradation of crystallizationaround the central part of substrate.

Referring to FIG. 4, there is shown a schematic diagram illustrating anexample of positional relationships of the substrate in relative to theintensity distribution at the time of laser beam scanning in themanufacturing method of an active matrix substrate in accordance withthe present invention. In this example a vertically elongate rectangularlaser beam of FIG. 4 is modulated periodically in the long axisdirection. Also in this figure there is shown an example of exposure bymoving the laser beam in the right hand direction of the figure at aconstant exposure interval 14. The position 15 at which the laser beamfluence is at maximum for each exposure is formed periodically in thelong axis direction, the position in this long axis direction man begiven by na+r, where n is an integer, laser beam modulation period 22 isa, a non-negative value smaller than a is r.

Since the maximum position 15 moves randomly in the long axis directionfor each exposure, value r will accordingly be changed. If the value ris smaller than the period a, the exposure position will besubstantially fixed, some area being irradiated by the laser beam ofweak fluence, arising a problem that the cristallinity decreases. Thefluctuating range 15 of the value r may be preferably at least a half ofthe period a or more.

Referring to FIG. 5, there is shown a schematic diagram illustrating themelting semiconductor film formed on a dielectric substrate constitutingan active matrix substrate when exposed to a pulsed laser beam. Thefollowing description will refer to a glass substrate for the dielectricsubstrate constituting the active matrix substrate. On the glasssubstrate 101, undercoat films 102 and 103 made of SiN and SiO by theplasma CVD method, and an amorphous silicon semiconductor film 2 for theprecursor are deposited at the thickness of 50 nm, 100 nm, and 50 nm,respectively. The amorphous silicon semiconductor film 2 aredehydrogenated by the annealing at 450 degrees, to avoid the developmentof defects caused by the hydrogen bumping at the time of laser beamexposure. A laser beam modulated after having transmitted through thephase shift mask 4 periodic to the long axis direction of the laser beamwill be exposed thereon. The laser beam will be weakened at the step ofthe phase shift mask 4, and emphasized at other zones. The laser beammodulation period 21 will be a half of the step of the phase shift mask4.

The amorphous silicon semiconductor film included in the zone 24 wherethe laser beam fluence exceeds beyond the melting threshold 23 will becompletely melted in the film thickness direction. On the other hand, inthe zone 25 where laser beam less than the melting threshold has beenexposed amorphous silicon semiconductor film will be partially meltedand crystallized to shift to polysilicon film. In the completely meltedzone 24 crystal will laterally grow up in the long axis direction aroundthe core of crystal of non-melted zone 25 in the vicinity. When settingthe pulse width of the pulsed excimer laser beam to 25 nsec, thethickness of the amorphous silicon semiconductor film 2 to 50 nm, themaximum lateral growth distance at the room temperature will beapproximately 1 μm. If the width of the completely melted zone 24 ismore than twice of the maximum lateral growth distance, the centerregion of the completely melted zone 24 will be left ungrown from thelateral growth, and unpreferably microcrystalline will be formed. Forexample, if the step period 21 of the phase shift mask 4 is 3 μm, themodulation period 22 of the laser beam is 1.5 μm, which is less thantwice of the lateral growth distance of 1 μm, even when the width ofcompletely melted zone 24 varies due to the fluctuation of laser beamfluence, microcrystalline development will be suppressed. This means anadvantage of a wider margin with respect to the fluence fluctuation ofthe laser beam. In the following description, the width of thecompletely melted zone 24 will be described to be less than twice of themaximum lateral growth distance.

Referring to FIG. 6, there is shown a schematic diagram illustrating anexample in a cross-section and plan view of a silicon film when the filmhas been crystallized by exposing with the laser beam modulated ofintensity by a phase shift mask and is exposed by the laser beammodulated again by the phase shift mask. The silicon film shown in FIG.6 is a polycrystalline film made of crystal grains 32, divided by theboundary 31 made by the previous exposure. The hillocks 33 along withthe volume change at the time of solidification are formed at the sameinterval as the interval of laser beam. On the area that has beenpreviously exposed by a laser beam of weak fluence, crystals 34 ofrelatively smaller grain size are formed. In this example, there isillustrated an exposure of the laser beam over the melting threshold 23to the crystal grain of relatively small size. The similar components ormeaning to FIG. 5 are designated to the identical reference numbers.

FIG. 7 shows a schematic diagram illustrating a cross sectional view anda plan view of crystals after exposing to the laser beam in the samearrangement as FIG. 6. The similar members are designated to theidentical reference numbers to FIG. 6. The crystals included in the zone24 exposed to the laser beam of more than melting threshold 23 in FIG. 6will be melted, and crystals in the same orientation as those at theedge of the completely melted zone 24 will grow laterally to form apolysilicon film 32 as shown in FIG. 7. The crystals at the both edgesof the completely melted zone 24 will have different crystalorientations in general, new crystal grain boundary 31 will be formed atthe center of the completely melted zone 24 of FIG. 7. On the center ofthe melted zone new hillocks 33 created is shown in FIG. 7. In thisembodiment, the crystals in the zone not included in the completelymelted zone 24 will grow laterally to form crystal grains 32 havingapproximately same length as the modulation period of the laser beam, sothat the polysilicon film will have the grain in the in the long axisdirection of the entire film almost matched to the modulation period ofthe laser beam.

Then by iteratively repeating the exposure of the laser beam while atthe same time moving randomly in the long axis direction, the meltedzone 24 at each exposure will move randomly. On the zone on which thelaser beam has been irradiated, lateral growth will develop to increasethe grain size in the long axis direction, eventually as shown in FIG.7, the grain size will become to the same length to the modulationfrequency of the laser beam. In addition, to obtain a goodcrystallization result, it is important that the grain size may not beshrunk in the succeeding exposure after the primary formation ofcrystals of the length the same as the period.

Referring to FIG. 8, there is shown an example of crystals prior to thelaser beam exposure of the energy over the melting threshold to thepolycrystalline silicon film having the grain size in the long axisdirection approximately as large as the period in such a manner as asingle crystal grain 32 not including the boundary is divided; FIG. 9shows an example of crystals after the laser beam exposure of the energyover the melting threshold to the polysilicon film having the grain sizeapproximately same as the period in such a manner as a single crystalgrain 32 not including the boundary is divided. Crystals before thelaser beam exposure has periodic hillocks 33 formed as shown in FIG. 8,and boundary 31 formed along with the hillocks 33. The crystals in themelted zone 24 will be melted by the laser beam exposure, however thecrystals at both sides are derived from one single crystal and theirorientation are the same so that the orientation of crystal afterlateral growth will be the same.

After laser beam exposure, as shown in FIG. 9, laterally grown crystalsin the center of the melted zone 24 will be fused to each other to formagain one single crystal 32. New hillocks 36 will be also formed aroundthe center of the melted zone 24, along with the increased volume. Thehillocks by the laser annealing are likely to be formed on theintersection of three or more crystals, and more than two boundarieswill be crossed over. On the other hand, as shown in the figure, theboundaries intersecting to the hillocks 36 of the semiconductor film ofFIG. 9 is characterized in that it includes hillocks including only oneboundary in the shape of letter Z. Since the width of crystals grownfrom both sides of the melted zone 24 is not the same, new boundary canappear around the center of the melted zone, however most of thecrystals grown from both sides will be fused without forming a newboundary, and thus a film having a grain size with its length almostidentical to the modulation period of the laser beam. Once the crystalgrains formed with the length almost identical to the modulation periodof the laser beam, the grain size after the succeeding laser beamexposure will be approximately constant. As a result, in accordance withthe preferred embodiment, the crystal grains of the length almostidentical to the modulation period will be eventually formed in a highprobability by a plurality of times of laser beam exposure.

In the crystallization method of a silicon semiconductor film inaccordance with the preferred embodiment, if there is a registrationerror between laser beam exposures, the error will be absorbed by therandom displacement added to the period direction of the laser beammodulation and canceled out. Therefore, the scanning error of the stagecarrying the substrate may be even larger than the lateral growthdistance, so that a simple stage scanning mechanism can be used todecrease the installation cost. Also, the exposure number of laser beammay be set such that on an average each zone will be melted once. Inthis embodiment for example ten exposures will form a crystal having thegrain size in the period direction of the modulation of laser beam,i.e., 1.5 μm. In comparison to the conventional multi-shot procedures ofthe prior art, which requires tens of exposures for forming a similarcrystal grain, the method in accordance with the present invention needsonly a fraction of exposures, resulting in a higher throughput.

The grain size of crystals can be determined by the modulation period ofa laser beam. Thus, almost the same size of crystal grains can beobtained even when the lateral growth distance is different and a largermargin to the laser beam fluence may be yielded. Also in the presentinvention entire film zone is not needed to be melted at once. Rather itis sufficient to emit the laser beam having a fluence beyond the meltingthreshold. This permits to having a lesser mean fluence. In themodulation by the phase shift mask, the mean intensity before and afterthe modulation is identical except for the loss by the reflex at themask, therefore laser output required to process can be furtherdecreased, in comparison to the conventional methods. When using a laserof the same oscillation power, a wider area can be treated at once,resulting in an improved throughput.

In accordance with the preferred embodiment, the displacement of theposition of laser beam exposure into the modulation period directionrequires a given zone to be emitted at a fluence beyond the meltingthreshold at least once. However the melted zone may or may not beoverlapped between two successive laser beam exposures. When theexposure position of laser beam is random each time, the probabilitythat there remains an area to which is never irradiated the laser beamof the fluence over the melting threshold value for a plurality ofexposures, will decrease exponentially with respect to the number ofexposures. When the modulation period of the laser beam is set to beless than twice of the lateral growth distance and the width of area toirradiate at a fluence over the melting threshold value is a half of theperiod, then crystal as approximately same as the period will be yieldedwith a number of shots as less as five, the throughput will bepreferably improved.

In accordance with the preferred embodiment, the positional change oflaser beam modulation toward the period direction may not be needed tobe quite random, but it is sufficient that the laser beam of the fluenceover the melting threshold be emitted to every point on the siliconsemiconductor film (Si film). For example, the moving distance of thelaser beam toward the modulation period direction can be set not to beconstant, so as to include a distance larger than a half of the periodamong shots. Also the laser beam can be oscillated at an amplitudelarger than the modulation period of the laser beam, at a perioddifferent from the integer of the number of shots. A good result can beyielded only if every point on the Si film are emitted once or twiceirrespective of the sequence. For example, it can be conceivable thatthe displacement direction in the period direction is reversed once ortwice for every a few shots. Also, the grain size of thus obtainedcrystals is not depended on the starting point of exposure so that noregistration is necessary, and it can be quite possible to crystallizeagain a substrate of insufficient crystallization to recover.

In the above-described embodiment, pulsed laser beam has been used,however a CW laser beam may also be used. In case of CW laser, thelateral growth position will move along with the displacement of theexposure point toward the modulation period direction, and the similarresult to the pulsed laser beam can be obtained. In addition thewavelength of laser beam may not be limited to 308 nm, any otherwavelengths can be used, provided that the beam is absorbed by the Sisemiconductor film to develop heat and that the phase shift mask isavailable, which is transparent and durable to the laser beam exposure.For example the excimer laser of the wavelength of 248 nm can be usedalong with a phase shift mask made of quartz. A CW laser of 532 nm ofwavelength may be used in combination with a phase shift mask formed bya transparent plastic. A plastic mask may be obtained in an inexpensiveway by mold forming. Since the plastic mask is superior shock resistantone, of which a thinner and lightweight mask can be made, which allowsspeeding up the movement to the above described period direction. Themanufacturing method of a polysilicon film in accordance with thepreferred embodiment can be used when the interference of laser beam isnot sufficiently high.

Referring to FIG. 10, there is shown a schematic diagram illustratingthe positioning of boundary and hillocks of polysilicon semiconductorfilm formed in accordance with the preferred embodiment of the presentinvention. In FIG. 10, a laser beam of the shape of very thin rectanglein the vertical direction of the figure in combination with a phaseshift mask periodic in the long axis direction is used to modulate thelaser beam to sequentially scan from left hand side to right hand sideof the figure to form a polysilicon semiconductor film. While movinghorizontally, that is in the short axis direction, the exposure point ofthe laser beam by an exposure interval 14 for every pulse emission oflaser beam, the laser beam will be randomly moved in the long axisdirection. The exposure interval 14 may be smaller than the width ofshort axis of the laser beam (not shown in the figure), and every pointof the semiconductor film will receive a plurality of number of laserbeam exposures.

For the semiconductor film, the hillocks 33 corresponding to the meltedzone at the exposure of laser beam will be formed as a linear array ofhillocks 38. The period 37 of the array of hillocks 38 is identical tothe modulation period of laser beam. Since the displacement of laserbeam toward the period direction is random, the position to which thehillock array will be formed will also be random, resulting in a randomerror 40 of the array of hillocks for every exposure interval 14. Thehillocks in a zone that receives a fluence larger than the meltingthreshold will be melted, however hillocks of any other zone will remainintact. As the width of melted zone may be smaller than the period, inmost cases arrays of hillocks not included in the melted zone of thelast laser beam exposure. Therefore other than the array of hillocksformed at the last laser beam exposure, there will remain other arraysof hillocks in the same period. This results in a plurality of arrays ofhillocks in one period.

After crystal grains of the length almost same as the period is formed,when a laser beam of the fluence larger than the melting threshold atthe position of dividing the grain is emitted to reform a singlecrystal, as shown in FIG. 9, an array of hillocks with only oneintersecting boundary will be formed. The semiconductor filmcrystallized by the random displacement of modulated position inaccordance with the preferred embodiment may have such a feature.

When the fluence of laser beam is low and melted zone width is smallerthan the modulation period of the laser beam, the array of hillocks asdescribed above may be likely to remain, and more arrays of hillocks inthe period tend to be created. The hillock closer than a half of thewidth of the melted zone may be fused together, the minimum distancebetween hillock arrays may be approximately equal to a half of thelateral growth distance of the last laser beam shot. For a film having aplurality of hillock arrays formed thereon, the hillock height tends tobe lowered due to the increased volume because the lateral growthdistance is small, such film is more preferable to be used as thin filmtransistor channel. When increasing the fluence of laser beam in thefirst half exposure to form crystal grains of the length approximatelyequal to the period with fewer number of shots with large width ofmelted zone, then decreasing the fluence of laser beam in the secondhalf exposure to narrow the width of melted zone, a plurality of hillockarrays can be made without making new boundary so as to decrease overallroughness, resulting in a semiconductor film suitable to be used as thinfilm transistor channel.

By preparing a fluence gradient such that the second half of exposurefluence is decreased in comparison with the first half of laser beamexposure fluence in the short axis direction, a similar effect can beobtained. By enlarging the modulation period of the laser beam in thefirst half of the laser beam exposure and shrinking the modulationperiod in the second half, a film having larger grain size correspondingto the modulation period of the laser beam of first half, and smallermean interval of hillock arrays corresponding to the modulation periodof the laser beam of second half, that is, the film having smallerroughness can be obtained.

Referring to FIG. 11, there is shown a microscope photographillustrating an exemplary Si film obtained by using the manufacturingmethod of the present invention. The polysilicon film shown in FIG. 11is consisted of a glass substrate, an undercoat film, and an amorphousSi film of 50 nm formed thereon, the amorphous Si film was thendehydrogenated and crystallized in accordance with the manufacturingmethod as above. The crystallization was done by means of pulsed excimerlaser beam modulated in the long axis direction with the width of 360 μmin the short axis direction and the width of 365 millimeters in the longaxis direction. Using a phase shift mask of 3-μm pitch, the laser beamwas modulated to 1.5-μm period. The exposure point was scanned from theleft to right of FIG. 11, while randomly moved in the long axisdirection. 10 shots of exposure were performed by setting fluence priorto shaping to a line beam to 800 mJ, and the displacement betweenexposing laser pulses in the short axis direction of the exposure pointto 36 μm. Thus obtained film was selectively etched the boundary byusing the Wright etcher, then observed on a scanning electron microscope(SEM). In FIG. 11 a film thicker than the circumference was deposited onmuch brighter area, and hillocks 33 were formed.

Referring to FIG. 12, there is shown a schematic sketch of boundarys andhillocks of FIG. 11. The periphery 37 of the array of hillocks 38linearly formed in FIG. 12 is 1.5 μm, matched to the modulation periodof the laser beam. A crystal grain 32 of the length almost equal to theperiod direction is formed as shown by shaded area. In a period anotherhillock array 39 is created, other than the array of hillocks 38 almostmatched to the boundary. The hillocks of the hillock array 39 include ahillock 36 having only one boundary.

Referring to FIG. 13 and FIG. 14, there are shown schematic diagrams ofprocess steps for manufacturing semiconductor films by exposing laserbeams each having a different periodicity. In FIG. 13, the laser beamwill pass through a phase shift mask 41 having a linear pattern ofperiod of 8 μm in the short axis direction of the linear shaped laserbeam 1. The phase shift mask 41 is held at the level of 0.7 millimeterabove the substrate, which creates laser beam having a periodicintensity distribution in the short axis direction of 4-μm period. Thefluence of the laser beam may be set such that the width of exposurezone of laser beam that exceeds the melting threshold of polysiliconfilm will be less than twice of the maximum lateral growth distance, forexample 1 μm when the maximum lateral growth distance is 1 μm.

The stage 5 carrying the glass substrate 6 may be scanned in the shortaxis direction during exposure, such that the laser beam of fluence overthe melting threshold is irradiated more than once on every area of thesubstrate. By setting the beam width to 330 μm, displacement of 33 μm inthe short axis direction for each exposure, 10 shots of laser beam wereexposed to the substrate having an amorphous Si film 2 formed. The widthof area exceeding the melting threshold in this preferred embodiment isapproximately a quarter of the laser beam modulation period, each areawill be melted more than once by those 10 shots. On the crystal grainsof the polysilicon film 42 after exposure the crystal grains will be inthe form of lateral growth in the short axis direction which is thedirection of intensity modulation 43 of the laser beam, and will havethe mean grain size of approximately 1 μm in the short axis direction,which size corresponds to the lateral growth distance.

When modulating the laser beam 1 using a phase shift mask 41 with aperiodicity in the short axis direction as is the present embodiment,the diffracted light in the short axis direction will escape out of theline shaped laser beam to introduce some loss. This loss will be largerwhen the pitch of phase shift mask 41 becomes smaller, and if thedistance from the phase shift mask to the substrate is larger. Thedistance between the glass substrate 6 and the phase shift mask 41 isrequired to be more than 0.2 millimeter, in the standpoint of preventingany debris onto the phase shift mask 41, and the distance may not be toocloser. Therefore in order to avoid the loss, for the phase shift mask41 with a period in the short axis direction, it is preferable that theperiod be larger than the phase shift mask of the period in the longaxis direction. The diffraction loss may be suppressed below 10%, apractical level, if the phase shift mask is pitched to more than 6 μmfor the laser beam shape and the phase shift mask height of thepreferred embodiment.

To suppress microcrystalline, the width of area exceeding the meltingthreshold will be limited to the maximum lateral growth distance. Whenthe modulation period of laser beam becomes larger, then the range oflateral growth in the exposure region of the laser beam will be narrowedand the number of exposure necessary to obtain a good crystallizationwill increase. In order to allow lateral growth of the entire surface inabout 10 shots, the modulation period of the laser beam needs to belimited to 5 times the maximum lateral growth distance. In the presentembodiment, for example 5 μm or less of laser beam period, entiresurface may be laterally grown by about 10 shots.

In FIG. 14, similarly to FIG. 1, laser beam 1 incident to a phase shiftmask 44 with a periodicity in the short axis direction is shaped to havea periodicity in the long axis direction, which is then emitted to apolysilicon film obtained by the method according to FIG. 13. Byirradiation of the laser beam while moving the stage 5 carrying theglass substrate 6 in the short axis direction at a predeterminedvelocity and giving a random displacement 46 in the long axis direction,a polysilicon film 45 having the crystal grains of the lengthapproximately equal to the modulation period of the laser beam in thelong axis direction may be obtained. In the polysilicon film 45, thegrain size in the direction perpendicular to the growth direction, i.e.,in the short axis direction, may vary depending on the crystalline ofthe polysilicon film 42, the grain size will become large in comparisonwith the direct exposure to the amorphous silicon film, resulting in afilm with anisotropy decreased.

Referring to FIG. 15, there is shown a microscopic photographillustrating an exemplary Si film formed by sequentially exposing thelaser beams each having the periodicity in the perpendicular directioneach to other in another embodiment of the manufacturing method inaccordance with the present invention. In this example a phase shiftmask of 10-μm pitch is used to modulate the laser beam in the short axisdirection of the beam, and 10 shots are exposed. Then the laser beam ismodulated in the long axis direction of the beam by using a phase shiftmask of 3-μm pitch to expose 10 shots to crystallize. The lattercrystallization was performed, by moving the substrate at a randomdistance for each shot, in the direction of modulation period of thelaser beam, i.e., in the long axis direction. FIG. 15 shows asemiconductor film thus obtained, etched with Wright etcher foreliciting the boundary, and observed by a scanning electron microscope(SEM).

Referring to FIG. 16, there is shown a schematic diagram depicting theboundary and hillocks of the photograph in FIG. 15. In FIG. 16, thehillocks 33 are formed at an interval of 1.5 μm, which is almostcorresponding to the modulation period of the laser beam of period inthe long axis direction exposed in the second half. Also plural arraysof hillocks 38 are formed in the period with the hillocks arrangedlinearly. The grain size perpendicular to the period is larger than thatdescribed with reference to FIGS. 11 and 12, corresponding to thelateral growth of crystallization by the laser beam of a first half,which is periodic in the short axis direction. The grain size wasapproximately 0.3 μm in FIGS. 11 and 12, and approximately 0.5 μm inaverage in this example, with decreased anisotropy. The hillock arraysin parallel to the long axis direction formed by the lateral growth inthe short axis direction are disappeared here due to the secondexposure, with no new roughness present with respect to the embodimentdescribed in FIGS. 11 and 12.

The present embodiment may be implemented by means of a single apparatusby adding a phase shift mask switching mechanism in the course ofmanufacturing process, without difficulties, with a lower implementationcost. A similar effect of decreased anisotropy may be yielded, bymodulating the laser beam using only a mask of periodicity in the longaxis direction, scanning the substrate in the short axis direction whilemoving randomly in the long axis direction, and turning the substrate 90degrees during the crystallization by the laser beam exposure, andcontinuing the exposure. In this case a mechanism for turning thesubstrate 90 degrees at the time of carrying in the stage or substrateis required, which implies a slight increase of installation cost,however crystals with anisotropy more decreased and approximately samegrain size in both long and short axis directions. In this method thehillocks formed in the first half will be almost disappeared, exceptthat mild hillocks will be remained by the laser beam exposure after arotation of 90 degrees, and the hillock arrays having the roughness inthe rotated direction approximately equal to the exposure withoutrotation of 90 degrees will be formed, so that there will be no increaseof roughness.

Referring to FIG. 17, there is shown a schematic plan view illustratingan exemplary circuitry on an active matrix substrate for an liquidcrystal image display device having thin film transistors formed on thepolycrystallized semiconductor film in accordance with the manufacturingmethod of the present invention. The liquid crystal display deviceincludes an active matrix substrate 51 having thin film transistorsformed thereon, and a liquid crystal 57 sealed between the substrate 51and an opposing substrate not shown in the figure. The active matrixsubstrate 51 having thin film transistors formed thereon is atransparent dielectric substrate (glass substrate) made of glass, aplurality of wirings (gate lines 52 and data lines 53) intersecting eachother placed thereon.

At an intersection of a gate line 52 and a data line 53, a pixel circuit54 is formed for driving the liquid crystal 57. The pixel circuit 54includes a pixel switch 55 composed of a thin film transistor served asa switch, and a storage capacitor 56. The thin film transistor served asthe pixel switch 55 may be the semiconductor film made in accordancewith the above-described manufacturing method of the present invention.Of the pixel driving circuitry, a gate line driver 58, which is acircuit for driving gate lines and a data line driver 59, which is acircuit for driving data lines are formed on a same glass substrate toform an active matrix substrate 51.

In accordance with the liquid crystal display device of the preferredembodiment, a large size semiconductor film may be formed on a glasssubstrate at a high throughput. This device will be suitable for theformation of thin film transistors used as pixel switches of an imagedisplay device, which is often required to have a large surface area. Inaddition the thin film transistors formed in accordance with the abovemanufacturing method may be incorporated to the pixel driver circuit inthe periphery of the pixel circuit that is required to be highperformance, because a semiconductor film having a relatively large sizeof crystal grain can be obtained. Since the semiconductor film inaccordance with the preferred embodiment may have in general largecrystal grain size in the modulation direction, that is the growthdirection, or in the long axis direction for example, the thin filmtransistor channel may have a higher electron mobility by beingarranging in the long axis direction.

When the area dedicated for peripheral circuitry (pixel drivers, etc.)is formed with thin film transistors on the glass substrate that formsthe active matrix substrate 51, it is preferable that the channeldirection of the thin film transistors of larger driving power may bedirected into the growth direction. On the other hand pixel circuit zonedoes not need relatively high mobility, so that the direction of thinfilm transistor channel may be placed in either the major or short axisdirection. Also, by using a process step for decreasing the anisotropyas shown in FIGS. 13 and 14, the performance of thus obtained thin filmtransistor may not be depending on the direction. Therefore thedirection of thin film transistor channel may be in either direction forthe circuitry.

Referring to FIG. 18, there is shown a schematic plan view illustratingan exemplary configuration of one pixel of a semiconductor device usedfor an liquid crystal display device formed by using the semiconductorthin film in accordance with the manufacturing method of the presentinvention. The liquid crystal display device may operate liquid crystalsandwiched between an active matrix substrate having thin filmtransistors formed thereon and an opposite substrate. In FIG. 18 onlythe active matrix substrate having thin film transistors formed thereonis shown.

Referring to FIG. 19, there is shown a cross sectional view taken alongthe dashed line A-B of FIG. 18. The polycrystalline Si film inaccordance with the manufacturing method of the present invention, isthe semiconductor film 104 having its periodicity in the long axisdirection, which is used for the thin film transistors forming the pixelswitch 55, and the storage capacitor 56. The capacitor 56 is connectedto the source or drain 61 of a thin film transistor of the pixel switch55, the other is connected to a pixel electrode 113 to maintain thevoltage applied to the pixel electrode 113. The pixel switch 55 is athin film transistor of double gate type, which has two gates. At thebottom end of gate 63 of the semiconductor film 104 an LDD 62 of n-typedomain, which is more lightly doped than the source or drain 61, isprovided to control the leak current. The reference numeral 53designates to a data line, 60 to a common line, 64 to a channel, 101 toa glass substrate, 102 to a SiN undercoat film, 103 to a SiO₂ undercoatfilm, 105 to a gate dielectric layer, 107 to an interlayer dielectriclayer, 112 to an organic protection film, and 113 to a pixel electrode.

The direction of the thin film transistor channel of the pixel switch 55is in the short axis direction, perpendicular to the period direction ofthe above described semiconductor film. Although the driving power isless than the thin film transistor having channel arranged in the longaxis direction, there may be a sufficient performance for driving theliquid crystal display. The capacitor 56 on the other hand will be usedby applying a positive voltage to the common line 60 to induce carriersto the underlying semiconductor film 104. By aligning the currentdirection to the direction in which the mobility of the semiconductorfilm is maximum, the resistance of the semiconductor film connected tothe capacitor 56 will decrease, allowing more rapid operation. In theexample shown in FIG. 18 the current will flow through the long axisdirection in the underlying semiconductor film 104 of the capacitor 56,such that the long axis is directed to the largest grain size.

Referring to FIGS. 20 to 25, there are shown schematic diagrams ofmanufacturing processes of the active matrix substrate shown in FIG. 19.At first, as shown in FIG. 20, On a glass substrate 101, 140 nmthickness of SiN undercoat film 102, 100 nm thickness of SiO₂ undercoatfilm 103, 40 nm thickness of amorphous Si film 114 are sequentiallydeposited by using plasma CVD method. After dehydrogenizing process ofthe amorphous Si film by the annealing at 400 C degrees 10 minutes,pulsed excimer laser is irradiated to crystallize. The crystallizationprocess uses laser beam of 1-μm period, modulated by a phase shift maskof 2-μm pitch, randomly moved in the modulation direction of the laserbeam. 10 shots of exposure may yield a polycrystalline Si 115 as shownin FIG. 21, consisted of crystalline of grain size of approximately 1 μmin the period direction.

The polycrystalline Si 115 will be processed by a photolithograph toshape as island, SiO₂ film of 130 nm thickness for a gate dielectricfilm 105 will be deposited by plasma CVD using TEOS (tetraethoxisilane)(see FIG. 22). A MoW (Molybdenum-Tungsten alloy) film 106 for the gatewill be sputtered on the gate dielectric film 105 to a thickness of 150nm. After forming MoW film 106, coating with a resist, mask exposure,and development will be sequentially performed to develop a resistpattern 116 (see FIG. 23). Using the resist as a mask, the MoW film 106will be etched with a mixture solution of phosphoric acid, nitric acid,acetic acid and water. As shown in FIG. 23, The MoW film 106 will beetched to a shape receded by 1 μm from the resist 116 to form a gate.Thereafter, P ions for n-type dopant will be implanted at theacceleration voltage of 60 KeV, at the doze of 10^15 per squarecentimeter, using the resist as mask, to dope into the Si film of thearea uncovered by the resist to form an n-type doped region 117 to yielda structure shown in FIG. 23.

After removing the resist, as shown in FIG. 24, a gate 63 will be usedas mask to implant P ions at the acceleration voltage of 70 KeV, at thedoze of 10^13 per square centimeter, to form an LDD 62 of the length of1 μm of self alignment at an end of the gate. Then, as shown in FIG. 25,interlayer dielectric film 107 of the thickness 500 nm will be depositedusing the plasma CVD with TEOS, annealed 1-hour 450 C degrees toactivate injected P ions to form the source or drain 61. Then aphotolithographic process will be performed using a hydrofluoric acidtreatment solution or dry etch of fluorides to pierce the contact holes.Thereafter, a sputter will be used to deposit an MoW film 108 forwirings, an aluminium alloy film 109 and an MoW film 110 to thethickness of 100 nm, 400 nm, and 50 nm, respectively. Then, a wet etchwill be performed to process these layer to wirings 65 to obtain astructure shown in FIG. 25.

Then, a protective layer (passivation layer) of SiN 111 of thickness 300nm will be formed using the plasma CVD, as shown in FIG. 19, then thesubstrate will be annealed 30 minutes 400 C degrees in the nitrogenenvironment to terminate the defects in Si film and defects of interfacebetween the Si film and gate dielectric film. Then an organic protectivelayer 112 shown in FIG. 19 will be applied. An exposure and developmentprocess will be then performed to form through holes used for theconnection to the pixel electrode 113, and responsive sputtering to formtransparent conductive layer made of ITO (Indium-Tin-Oxide) for thepixel electrodes. A photolithographic process step will be thenperformed to form pixel electrodes 113 to obtain an active matrixsubstrate having thin film transistors formed thereon as shown in FIG.19. Thereafter, a directive layer for directing liquid crystal to theupper layer of the pixel electrodes will be applied thereon anddirective controllability will be added by rubbing or light exposure.Finally liquid crystal will be sealed between an opposite substratehaving a color filter formed and the substrate to obtain a liquidcrystal display device.

In this embodiment, only formation of n-type thin film transistor hasbeen shown, however an active matrix substrate having CMOS thin filmtransistor circuit, including both n-type and p-type thin filmtransistors can be made by adding process steps of masking with a resistany necessary regions, and implanting Boron ion instead of Phosphor ion.Also, in this embodiment dielectric substrate of the active matrixsubstrate has been made of glass plate, however the dielectric substratemay not be limited thereto, any other suitable materials includingplastics and isolated metallic plate can be used instead.

For a plastic substrate, which is likely to have some lateral size errorcaused by the thermal inflation by the heat of laser beam exposure aprecise registration may be required such as SLS method of the priorart, however this may cause defects of film because of registrationerror. The manufacturing method of semiconductor thin film in accordancewith the present invention does not require any registration, may beapplied without difficulties to such a plastic substrate. In accordancewith a preferred embodiment of the manufacturing method, the presentinvention may have advantages that mean fluence may be decreased incomparison with ELA method, which does not use a phase shift mask, andthat damages to the substrate may be decreased since a fewer number ofshots may yield a comparable crystal.

Referring to FIG. 26, there is shown an exemplary image display deviceusing organic electroluminescence elements using the active matrixsubstrate made by the manufacturing method in accordance with thepresent invention. An image display device using organicelectroluminescence elements (organic electroluminescence displaydevice), as similar to liquid crystal display devices, may have aplurality of intersecting wirings and pixels in the vicinity ofintersections formed on a dielectric substrate such as a glass plate. InFIG. 26, there is shown a schematic plan view of one exemplary pixel.The organic electroluminescence device in accordance with the preferredembodiment emits light through the substrate, requiring cathodes made ofAl alloy on the top surface. In FIG. 26 these cathodes are not indicatedfor the sake of clarity.

Referring to FIG. 27, there is shown a schematic diagram of equivalentcircuitry of an organic electroluminescence element, in which thereference numerals identical to those in FIG. 26 designate to the sameor similar components. FIG. 28 shows a cross sectional view taken alongthe line C-D of FIG. 26. In FIG. 28, the semiconductor film 104 having aperiodicity in the long axis direction, manufactured in accordance withthe manufacturing method of the present invention, is formed on a glasssubstrate 101 to form an n-type thin film transistor 72 having LDDcreated. The reference numeral of the undercoat layer on the glasssubstrate 101 is not shown in the figure. The semiconductor film 104having a periodicity in the long axis direction, manufactured inaccordance with the manufacturing method of the present invention, isformed on a glass substrate 101 to align the channel direction and theperiodicity direction to form a p-type thin film transistor 73 thereonwith the length of gate being a periodicity times a multiple of naturalnumber. The n-type thin film transistor 72 will be turned on by thevoltage applied to the gate line 52 to charge the capacitor 56 with thesignal voltage of the data line 53.

After charging, gate line voltage will turn off the n-type thin filmtransistor 72 to retain the signal voltage in the capacitor 56. Atransparent electrode 75, which is connected to the p-type thin filmtransistor 73, has an organic electroluminescence element 74 includingan organic film 118 composed of a hole transfer layer, light emittinglayer, and electron transfer layer, and a cathode 76 of Al alloydeposited on the entire surface. The gate 63 of the p-type thin filmtransistor 73 is applied with a voltage retained in the capacitor 56,which controls the current flowing through the organicelectroluminescence element 74 to a potential corresponding to theretained voltage to adjust the amount of light emission of the organicelectroluminescence element 74.

The semiconductor film on an active matrix substrate, made by themanufacturing method of an active matrix substrate in accordance withthe present invention, may be a film with a constant grain size in theperiodicity direction. When forming a thin film transistor that flowscurrent through the periodicity direction, the fluctuation of currentmay be decreased. In particular, when forming a thin film transistorwith a channel of the length of the periodicity times an integer, thenumber of grain boundaries included in the channel will be constant toallow improvement of homogeneity. Therefore in a device in which theimage display may be affected by the current fluctuation of thin filmtransistor driving the light emitting element, such as an image displaydevice using the organic electroluminescence element, the homogeneity ofdisplayed image may be improved by forming the semiconductor film of thepresent invention for the channel, forming the channel in theperiodicity direction of the semiconductor layer, and using a thin filmtransistor for driving the organic electroluminescence element with thechannel length of an integer times the periodicity.

Although in the above described preferred embodiments, there has beendescribed a manufacturing method of an active matrix substrate includedin an image display device such as liquid crystal display device andorganic electroluminescence element, as well as an image display deviceusing the active matrix substrate, it should be understood by thoseskilled in the art that the present invention may not be limitedthereto. The present invention may also be applied equivalently to anyother various semiconductor devices using a dielectric substrate such assemiconductor film wafer.

In accordance with present invention, a semiconductor film for forming asemiconductor device such as a high performance thin film transistor maybe manufactured at lower cost and higher throughput. A semiconductordevice including a high performance active matrix substrate thereby maybe manufactured at lower cost, enabling to provide an image displaydevice, which may display a high quality image, by incorporating theactive matrix substrate of the present invention as a component.

1. An image display device including an active matrix substrate made ofa dielectric substrate having a pixel circuit with a matrix array of anumber of pixels and a pixel driver circuit located outward from saidpixel area, said active matrix substrate comprising: a polycrystallinesemiconductor film deposited on said dielectric substrate, wherein aplurality of linear arrays of hillocks are located on a surface of thepolycrystalline semiconductor film in a direction parallel to eachother, and intervals between the linear arrays are random.
 2. An imagedisplay device set forth in claim 1, further comprising: a thin filmtransistor using said polycrystalline semiconductor film for a channelthereof.
 3. An image display device set forth in claim 2, furthercomprising: a plurality of mutually intersecting wirings formed on saiddielectric substrate; a pixel formed in the vicinity of saidintersection of wirings to vary the transmittance or reflectance oramount of light emission; and a thin film transistor formed in saidpixel for serving as a switch for selecting said pixel, wherein saidthin film transistor is formed with a channel using said polycrystallinesemiconductor film.
 4. An image display device set forth in claim 2,further comprising: a plurality of mutually intersecting wirings on saiddielectric substrate; a pixel formed in the vicinity of saidintersection of wirings; a light emitting element having an organic filmformed within said pixel; a thin film transistor formed within saidpixel for driving said light emitting element; wherein a channeldirection of said thin film transistor is parallel to a periodicdirection of the hillocks of said polycrystalline semiconductor film anda length of a channel of said thin film transistor is a multiple of anatural number of the periodicity of the hillocks of saidpolycrystalline semiconductor film.